In the manufacture of artificial dental crowns and bridges from plastic-faced metals, the metal framework is covered by a non-transparent, opaque material to keep the metal substrate from influencing the color of the plastic facing and to improve the bonding between the plastic facing and the metal. Such coatings are generally referred to in the art as opaque base masses.
U.S. Pat. No. 3,470,615 discloses a material adapted for this purpose which is produced from titanium dioxide, barium sulfate, aluminum oxide and polymer, dissolved in polyglycol dimethacrylate. The resulting resin is coated on the metal substrate and then hardened by heating for 2 to 20 minutes.
A synthetic resin mixture containing a copolymer of acrylonitrile and methyl methacrylate, pigment and an organic solvent (e.g. nitroparaffin and halogenated hydrocarbons) is disclosed in GB No. 1,418,004. The resin mixture provides an opaque layer on the substrate (e.g. metal crown) and is coated thereon by dissolving the copolymer in a solvent (e.g. nitromethane), dispersing the pigment in the solution, applying the solution to the substrate and permitting the organic solvent to evaporate. The pigments disclosed in the reference include silica, titanium dioxide, zinc oxide, iron oxide, barium sulfate, and mixtures thereof.
The prior art compositions are disadvantageous because they take a relatively long time to harden and/or require the use of organic solvents which must be removed after coating.
It is therefore an object of the present invention to provide a pigment-containing dental opaquing composition which does not require the use of organic solvents.
It is a further object of the present invention to provide a pigment-containing dental opaquing composition which rapidy hardens on a substrate such as a metal crown.